You can express yourself better if you learn the proper words to use for each situation.
You can pick up these words by reading good books and articles. Just be careful you don't pronounce something incorrectly in your head, and then speak that way in public. People will think you're ignorant.
I remember listening to a radio talk show one time when a man called in and mispronounced a word. The guest, who disagreed with him, attacked his mispronunciation, and the host was clearly embarrassed for him. All in all, it was just an awkward moment. And you definitely don't want to be initiating awkward moments while trying to drum up business.
Pronunciations do vary depending on your locale, so you could just say that's how it's pronounced where you're from. But there are usually only a few alternatives, and most educated people know of them.
You can learn proper pronunciation by listening to intelligent people. If intelligent people are rare where you live, buy some tapes or visit some podcast directories.
Some well-read people mispronounce words they read all the time but never hear. If you found a great word in a book that you're not sure about, check it out at Dictionary.com. They have a pronunciation guide, and, if you want to, you can sign up for their premium service. They have a feature where you can click on a word and hear the proper pronunciation.
At any rate, just make sure you know how the word is pronounced and what it means, before you use it. Nothing sounds worse than someone using big words out of context. There's nothing wrong with using big words, though, as long as you're using them correctly.
Of course, you should probably stick with the shorter, more common alternative if one exists. Don't use a big word just because you know it. Only use a bigger word if it's the only word available to express exactly what you mean. Most people have very small vocabularies, and will tune you out if you start talking over their heads.
If you're talking with experts, you'll probably want to use shortcuts (jargon). This can save time. Just don't use jargon outside specific groups, because it'll sound like gibberish to most people. You can learn this jargon by reading industry-specific journals and visiting message boards.
Speaking precisely isn't that hard. Just use the right word at the right time. That knowledge will only come with experience.